HIV-1 encodes a number of genes that are crucial for replication in primate cells. Gag, Pol, and Env products represent the main virion components, while Tat and Rev products regulate intracellular transcriptional and post-transcriptional events for the controlled expression of viral genes. Of particular interest are the HIV accessory proteins Vif, Vpr, Vpu, Vpx, and Nef, which are unique to primate lentiviruses. There is increasing evidence that these proteins operate in conjunction with specific host factors. In fact, most if not all of the accessory proteins appear to lack catalytic activities but instead seem to function as adaptors to link viral or cellular factors to pre-existing cellular pathways. In FY 2003, we completed and published a study investigating the regulation of Vpu and Vif expression, which is strictly dependent on the activity of the viral Rev protein. We found that Vpu and Vif mRNAs are inherently unstable in the absence of Rev and are rapidly degraded. Codon-optimization of these genes increased the stability of Vif and Vpu mRNAs and allowed for the Rev-independent expression of these proteins. In addition, we completed and published a study demonstrating the effect of Vif on a cellular inhibitor of HIV, APOBEC3G. The main conclusion from that study is that Vif regulates viral infectivity by inhibiting the encapsidation of APOBEC3G into viral particles. Finally, we completed and published a study demonstrating that Vif interacts with the viral Gag precursor and is able to modulate the maturation of viral capsid proteins. Aside from these completed and published studies, we initiated several new projects that aim at the characterization of APOBEC3G. One of these studies investigates post-translational events, in particular protein-folding or homo- and hetero-oligomerization of the protein and their effects on APOBEC3G function. A second study aims at the in vitro characterization of APOBEC3G to define protein-protein and protein-nucleic acid interactions involving APOBEC3G. Last but not least, we initiated a project to study the function of SIV and HIV-2 Vif proteins in the context of human or simian APOBEC proteins. From these studies we hope to get a better understanding of the function of viral accessory proteins such as Vif with respect to their manipulation of cellular factors.